Working out regularly may brighten the mood of people with chronic health problems like cancer, heart disease and back pain, according to the first sweeping look at previous research.
But it's no miracle cure: On average, six people would need to hit the gym or go for a jog for one person to see a mood improvement.
"It's a nice piece of evidence and I'm pleased because I like the concept," said Dr. Alan J. Gelenberg, who chairs the department of psychiatry at Penn State University in Hershey.
Gelenberg, who wasn't involved in the new work, said the findings jibe with guidelines from the American Psychiatric Association, which recommends regular exercise against the blues.
"There is some evidence for its use to prevent depression, and there actually is evidence for exercise as a treatment in itself," he told Reuters Health.
With the new study, published in the Archives of Internal Medicine, researchers wanted to weigh the evidence that training can also help chronically ill people who don't have a diagnosis of depression, but nonetheless may feel down.
That's important because depressive symptoms could make people less likely to take their meds, could increase their use of health services and decrease their quality of life, said Matthew Herring of the University of Alabama at Birmingham.
He and his colleagues combed through 90 previous studies including more than 10,000 people with health problems like cancer, heart disease, chronic obstructive pulmonary disorder (COPD), fibromyalgia, chronic pain or obesity.
In each study, people had been randomly chosen to do exercises -- on average, three times a week over 17 weeks -- or not.
According to Herring, people's depressive symptoms, as rated on a variety of psychological scales, dropped about 22 percent with exercise overall. That's similar to the effects on fatigue, anxiety, pain and other mental health outcomes.
"The magnitude of the effect of exercise training on depressive symptoms among patients found in our review is small but significant," he told Reuters Health by email.
Herring added that moderate -- at least 150 minutes of moderate intensity exercise per week -- and vigorous -- at least 75 minutes of vigorous intensity exercise per week -- seemed to help the most.
How long will benefits last?
Still, the report comes with several caveats. For instance, it's not clear how many people with chronic illnesses are able to work out at sufficient intensity, and many participants did in fact drop out of the studies.
Also, it's not clear how long the effects last, how much to exercise and what kind of exercise works better -- aerobic training like running or walking or strength training like weightlifting.
"What we don't know is much more than we do know," said Gelenberg.
Still, he added, "exercise has a lot of benefits... if someone doesn't exercise in a stupid way, like a 65-year-old man trying to bench press 200 pounds."
Gelenberg said people with chronic disease who feel depressed should exercise within a physician's guidelines and eat a healthy diet.
"I would suggest they indulge themselves in healthy pleasures: people, books, walks, sitting in a pretty place. If they still feel 'down,' I'd suggest professional attention to consider psychotherapy or an antidepressant medicine," he said.
(Corrects duration of exercise interventions in paragraph 9 of story posted on Jan 25, 2012)